1. Field of the Invention
The present invention is directed to a pressure sensor which uses elements operating with acoustic surface waves (SAW elements) wherein a reflective saw delay line with at least three reflectors is positioned on a pressure-sensing membrane such that it extends over both an expanding region and a compressing region of the membrane.
2. Description of the Prior Art
Pressure sensors of this generic type are known, for example, from the 1994 publication "IEEE International Frequency Control Symposium," pages 395-400. In a known SAW pressure sensor of this type, two SAW resonators are provided on a piezoelectric substrate fashioned as a pressure-sensing membrane; one sensor being arranged in the center of the pressure-sensing membrane and the other being arranged outside of such center. The two resonators are connected into the feedback loop of two oscillators wherein they serve as frequency control elements. The deformation that arises when pressure is exerted on the membrane causes changes in the mid-frequencies of the resonators. Since the resonance spaces of the resonators are located at points of opposite signs of the deformation, their respective frequency displacements likewise have different signs. The difference in frequency is the sum of the resonator frequency displacements. The measured pressure is a function of this difference in frequency and a function of the temperature. For the compensation of temperature changes, a thermoresistor is provided in addition to the SAW resonators.
Pressure sensors of this type require the incorporation of active electronic wiring of the SAW resonators for the realization of oscillators. In addition, pressure sensors of this type are not unproblemmatically remotely interrogatable. As such, the possibilities of their use are limited. For example, pressure sensors of this type can be used at wire bound points that are difficult to reach or that are completely inaccessible only with difficulty, if they can be used here at all.